1. Field of the Invention
The present invention relates to a holographic recording/reproducing apparatus which either records or records and reproduces information holographically, and also relates to a reproducing apparatus which reproduces holographically recorded information.
2. Description of the Related Art
In a typical holographic recording system, a laser beam from a laser source is split along two optical paths. One of the split laser beams is spatially modulated by a spatial light modulator in accordance with information to be recorded, and the thus modulated laser beam serves as a signal beam. The other one of the split laser beams serves as a reference beam. The signal beam and the reference beam are superimposed on a holographic recording medium so that the two laser beams interfere with each other and an interference pattern is formed on the holographic recording medium as a refractive index variation. Thus, the information is recorded on the holographic recording medium.
When the thus recorded information is reproduced, the signal beam is blocked and only the reference beam is directed onto the holographic recording medium at the same incident position and incident angle as those in the recording process. A diffracted beam corresponding to the original signal beam is obtained from the interference pattern on the holographic recording medium as a reproduction beam, and the reproduction beam is detected by a charge coupled device (CCD) sensor or the like.
The above-described holographic recording system can also perform multiplexed holographic recording. Multiplexed holographic recording is performed by, for example, an angular multiplexing method in which the incident angle of the reference beam on the holographic recording medium at the position where the signal beam is directed is varied or by a shifting method in which the reference beam is shifted in the multiplexing direction.
FIG. 14 is a schematic diagram showing the construction of a known holographic recording/reproducing apparatus. In this apparatus, a laser beam L from a laser source 101 is split into two laser beams L1 and L2 by a beam splitter 102. The laser beam L1 is guided through a shutter 103 and a lens system 104 into a polarization beam splitter (PBS) 105, and is incident on a reflective spatial light modulator (SLM) 106. The laser beam L1 is spatially modulated by the spatial light modulator 106 in accordance with information to be recorded, and the thus modulated laser beam L1 serves as a signal beam. The signal beam passes through the polarization beam splitter 105 and a first lens system 107, which serves as a Fourier-transform (FT) lens, and is directed onto a holographic recording medium 108.
The other laser beam L2 split by the beam splitter 102 is reflected by a mirror on a so-called page motor such that the reflection angle can be changed, and is focused onto the holographic recording medium 108 by a second lens system 110 as a reference beam. Accordingly, the signal beam and the reference beam interfere with each other and the information is recorded holographically.
When the thus recorded information is reproduced, the signal beam is blocked by closing the shutter 103 and only the laser beam L2, which serves as the reference beam, is directed onto the holographic recording medium 108 along the same optical path as that in the recording process. Accordingly, a reproduction beam corresponding to the recorded information is extracted from the back of the holographic recording medium 108, that is, from the side opposite to that on which the signal beam is incident. The reproduction beam is guided through a third lens system 111, which serves as an inverse Fourier-transform (IFT) lens, and is detected by a CCD sensor 112 or the like (refer to, for example, “Holographic Data Storage” edited by H. J. Coufal, D. Psaltis, and G. T. Sincerbox, Springer Series in Optical Sciences, Springer Verlag, July 2000, p. 350). Thus, a transmissive holographic recording medium is used as the holographic recording medium in this apparatus.
In the above construction, each of the first, second, and third lens systems 107, 110, and 111 has a complex lens structure including a lens group obtained by combining a plurality of lenses in order to increase the field of view and improve the focus performance, and therefore the apparatus is large and expensive.
Apparatuses using a holographic recording medium having a reflective film are also disclosed (refer to, for example, Optical Data Storage 2001, Proceedings of SPIE Vol. 4342 (2002), p. 567).
FIG. 15 is a schematic diagram showing the construction of another known holographic recording/reproducing apparatus which uses a holographic recording medium having a reflective film. In this apparatus, a laser beam L from a laser source 201 is expanded by a beam expander 202 and is split into two laser beams L1 and L2 by a beam splitter 203. The laser beam L1 passes through a beam shaper 204, is reflected by a mirror 205, is guided into a spatial light modulator (SLM) 206, and is modulated in accordance with information to be recorded, and the thus modulated laser beam L1 serves as a signal beam. The signal beam is reflected by a polarization beam splitter 207, passes through a quarter-wave plate 208, is focused by a first lens system 209, which is a lens group serving as a recording/reproducing lens system, and is reflected by a mirror 210. The reflected signal beam passes through the lens system 209 and the quarter-wave plate 208 again so that its plane of polarization changes. Accordingly, the signal beam passes through the polarization beam splitter 207. Then, the signal beam passes through a polarization beam splitter 211, a quarter-wave plate 212, and a second lens system 213, which is a lens group serving as an objective lens, and is directed onto a holographic recording medium 214.
The other laser beam L2 split by the beam splitter 203 is used as a reference beam. The laser beam L2 passes through a beam shaper 215, where the shape of the laser beam L2 is adjusted, is guided into the polarization beam splitter 207 via a mirror 216, a beam contractor 217, a mirror 218, and a Fourier-transform (FT) lens 219, is reflected by the polarization beam splitter 207, passes through the polarization beam splitter 211, the quarter-wave plate 212, and the second lens system 213, and is directed onto the holographic recording medium 214 at the same position as the signal beam.
Accordingly, the signal beam and the reference beam interfere with each other and the information is recorded on the holographic recording medium 214.
When the thus recorded information is reproduced, the signal beam is blocked and only the laser beam L2, which serves as the reference beam, is directed onto the holographic recording medium 214 along the same optical path as that in the recording process. Accordingly, a reproduction beam corresponding to the signal beam is extracted from the front of the holographic recording medium 214, that is, from the same side as that on which the signal beam is incident. The reproduction beam is guided through the second lens system 213 and the quarter-wave plate 212 into the polarization beam splitter 211, is reflected by the polarization beam splitter 211, and is detected by a CCD sensor 211 or the like via a polarizer filter 220.
In the above apparatus which uses a reflective holographic recording medium, that is, a holographic recording medium having a reflective film, the first and second lens systems have a large and complex lens structure. Therefore, compared to the apparatus shown in FIG. 14 which uses a transmissive holographic recording medium, the structure of the entire lens system can be simplified.
However, in this construction, the reference beam reflected by the holographic recording medium 214 is also received by the sensor 221 in the reproduction process. Therefore, the signal-to-noise (S/N) ratio decreases due to the increase in noise and the efficiency of sensor elements also decreases, which results in lower recording density.